Availability Zones

In Amazon Web Services (AWS), you generally spread your nodes over multiple availability zones for high availability. Unfortunately, not every node type is available in every availability zone, and in general, it is hard to know which zones one is available in in advance.

Error Types

If you are provisioning a single EC2 instance or you are only provisioning EC2s in an Auto Scaling Group (ASG) in a single-zone, you will obviously notice if you chose an incompatible zone for your instance type as it just won’t work.

It can be more nefarious when you have an ASG with multiple zones though. For example, our large scale airflow service runs in kubernetes, and the main ASG goes over 3 zones. Today, we ran out of IPs in two zones and realized that the third was not even being utilized. When hunting down why, this message was in the “activity” tracker page for the ASG.

Launching a new EC2 instance. Status Reason: Your requested instance type (r5.2xlarge) is not supported in your requested Availability Zone (us-east-1e). Please retry your request by not specifying an Availability Zone or choosing us-east-1a, us-east-1b, us-east-1c, us-east-1d, us-east-1f. Launching EC2 instance failed.

This is a very helpful message, but it’s unfortunate that we had to do the wrong thing in order to get the supported zones list.

Getting the Correct Zones in Advance

You can use this AWS CLI (V2) command to check the list of zones supported for an instance type in advance.

% aws ec2 describe-instance-type-offerings --location-type availability-zone --filters="Name=instance-type,Values=r5.2xlarge" --region us-east-1 --output table
-------------------------------------------------------
|            DescribeInstanceTypeOfferings            |
+-----------------------------------------------------+
||               InstanceTypeOfferings               ||
|+--------------+--------------+---------------------+|
|| InstanceType |  Location    |    LocationType     ||
|+--------------+--------------+---------------------+|
||  r5.2xlarge  |  us-east-1f  |  availability-zone  ||
||  r5.2xlarge  |  us-east-1c  |  availability-zone  ||
||  r5.2xlarge  |  us-east-1b  |  availability-zone  ||
||  r5.2xlarge  |  us-east-1d  |  availability-zone  ||
||  r5.2xlarge  |  us-east-1a  |  availability-zone  ||
|+--------------+--------------+---------------------+

Sources

You can find some information on this from AWS at this link.